Not being a servicer, yes on very long runs you could run dry, which will kill your element quickly, but I think theres enough water in there to make quite a bit of steam. A more likely option (only based on reading this site, not your usage patterns which I dont know) is that they are not primed after steaming, which lead them to boil dry. Do you always prime it or do you use another method of telling when the water is low? Is there a chance it could have been left on at times with low water? Is the steam wand dripping? This can empty the boiler before you realise.